Optimizing the hydraulic performance of a baffled horizontal subsurface flow constructed wetland through computational fluid dynamics modelling

Baffled constructed wetlands (CWs) offer a promising solution to address low hydraulic efficiency in traditional CWs. However, there is a research gap in the field regarding the optimal length and quantity of baffles, and their comprehensive effects on hydraulic efficiency. This study is the first CFD-based assessment to comprehensively investigate the combined influence of baffle length and the number of baffles on the hydraulic efficiency of CWs. Using OpenFOAM simulations at a laboratory scale, various baffle configurations were examined with lengths ranging from 0.4 m to 0.58 m and baffle numbers varying from 0 to 11. Experimental tracer tests were conducted to validate the simulations. The high correlation coefficient (R2) between the tracer test results and simulations (ranging between 0.84 and 0.93) further underscores the reliability of the findings. Residence time distributions (RTDs) were derived from the temporal evolution of the outlet concentration of a tracer. The results indicate that augmenting the number of baffles under a fixed baffle length has a greater impact on the RTD curves, causing a backward displacement of the peak time. However, when the number of baffles is three or fewer, extending the baffle length does not significantly affect the RTD. When the baffle length is held constant at 0.58 m, there is a 58% enhancement in hydraulic efficiency as the number of baffles increases from 0 to 5. However, when maintaining a constant number of 11 baffles, increasing the baffle length from 0.4 to 0.5 m results in only a 5.5% improvement in hydraulic efficiency. Moreover, a generalized predictive equation for hydraulic efficiency was derived based on the CFD results and dimensional analysis. The study enhances the optimization of constructed wetland design by providing greater understanding of hydrodynamic behavior, leading to improved performance and applicability in practical environmental engineering.